manual: Unify EXAMPLE title in code and manual

There are 3 types of EXAMPLE title in manual and code:
1: "# EXAMPLE"
   runc-delete.8.md
   runc-exec.8.md
   runc-kill.8.md
2: "EXAMPE:"
   runc-spec.8.md
3: EXAMPLE title exist in manual, but not exist in code's --help output
   delete.go
   exec.go
   kill.go

This patch unified above format, and deleted some useless blanks.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

delete.go

@@ -16,7 +16,8 @@ var deleteCommand = cli.Command{
 	ArgsUsage: `<container-id>
 
 Where "<container-id>" is the name for the instance of the container.
-	 
+
+EXAMPLE:
 For example, if the container id is "ubuntu01" and runc list currently shows the
 status of "ubuntu01" as "destroyed" the following will delete resources held for
 "ubuntu01" removing "ubuntu01" from the runc list of containers:  

exec.go

@@ -22,6 +22,7 @@ var execCommand = cli.Command{
 Where "<container-id>" is the name for the instance of the container and
 "<container command>" is the command to be executed in the container.
 
+EXAMPLE:
 For example, if the container is configured to run the linux ps command the
 following will output a list of processes running in the container:
 	 

kill.go

@@ -56,7 +56,8 @@ var killCommand = cli.Command{
 
 Where "<container-id>" is the name for the instance of the container and
 "<signal>" is the signal to be sent to the init process.
-	 
+
+EXAMPLE:
 For example, if the container id is "ubuntu01" the following will send a "KILL"
 signal to the init process of the "ubuntu01" container:
 	 

man/runc-spec.8.md

@@ -14,7 +14,7 @@ parameter that is initially set to call the "sh" command when the container is
 started. Calling "sh" may work for an ubuntu container or busybox, but will not
 work for containers that do not include the "sh" program.
 
-EXAMPLE:
+# EXAMPLE
   To run docker's hello-world container one needs to set the args parameter
 in the spec to call hello. This can be done using the sed command or a text
 editor. The following commands create a bundle for hello-world, change the